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RFC 1832 - 3.15 Discriminated Union

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### 3.15 Discriminated Union

A discriminated union is a type composed of a discriminant followed by a type selected from a set of prearranged types according to the value of the discriminant. The type of discriminant is either "int", "unsigned int", or an enumerated type, such as "bool". The component types are called "arms" of the union, and are preceded by the value of the discriminant which implies their encoding. Discriminated unions are declared as follows:

             union switch (discriminant-declaration) {
             case discriminant-value-A:
                arm-declaration-A;
             case discriminant-value-B:
                arm-declaration-B;
             ...
             default: default-declaration;
             } identifier;

Each "case" keyword is followed by a legal value of the discriminant. The default arm is optional. If it is not specified, then a valid encoding of the union cannot take on unspecified discriminant values. The size of the implied arm is always a multiple of four bytes.

The discriminated union is encoded as its discriminant followed by the encoding of the implied arm.

               0   1   2   3
             +---+---+---+---+---+---+---+---+
             |  discriminant |  implied arm  |          DISCRIMINATED UNION
             +---+---+---+---+---+---+---+---+
             |<---4 bytes--->|

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Connected: An Internet Encyclopedia
RFC 1832 - 3.15 Discriminated Union